3d printing of thermoset polymers and composites

ABSTRACT

The present disclosure provides a novel method of 3D printing using frontal polymerization chemistry. This method enables the printing of tough, high quality thermosets in a short time with the option of adding fiber reinforcement. As such, it facilitates fabrication of mechanically robust 3D-printed devices and structures.

RELATED APPLICATIONS

The present patent document is a division of U.S. patent applicationSer. No. 15/979,029, filed on May 14, 2018, which claims priority under35 U.S.C. § 119(e) to U.S. Provisional patent Application No.62/506,167, filed May 15, 2017. Both of the aforementioned patentapplications are hereby incorporated by reference in their entirety.

GOVERNMENT SUPPORT

This invention was made with government support under Grant No.FA9550-16-1-0017 awarded by the U.S. Air Force and DE-ACO2-06CH11357awarded by the U.S. Department of Energy. The government has certainrights in the invention.

BACKGROUND OF THE INVENTION

Current 3D printing techniques are generally only amenable tothermoplastics and low-quality thermoset resins. Those that can producestructural, high quality thermosets require a post-printing cure stepthat requires a number of hours for curing.

Common 3D-printing techniques such as fused deposition modelling (FDM)and stereolithography (SLA) are known to produce materials that havepoor mechanical properties. In FDM, a thermoplastic filament is extrudedthrough a heated print head in a defined pattern to produce a 3D partlayer by layer. However, the layer by layer method of deposition enablesdelamination between polymer layers in the final part. Furthermore, thethermoplastic polymers used for 3D printing do not typically possess thestiffness and strength required for high performance applications.

The SLA printing technique uses ultraviolet light to cure aphotosensitive resin in a defined pattern by projecting the pattern intoa bath of liquid resin. However, there are a limited number of polymerchemistries amenable to SLA, and the polymers that can be so producedare typically too brittle for engineering applications. Furthermore, therequirement for light to penetrate the sample precludes the use ofoptically opaque reinforcing materials.

Given these limitations, it is highly difficult to fabricate continuousfiber-reinforced thermoset composites via 3D printing. One method thathas been adopted is the extrusion of a partially cured resin containinga continuous fiber reinforcement. The material is essentially a B-stageresin, which is traditionally cured during printing using the constantapplication of UV-light at the print head or after printing using aconventional thermal cure cycle. Applying UV-light during printing ischallenging, since building on layers requires the resin to be fullycured before the print head advances. This slows the process andrequires additional lighting equipment to facilitate. Adding aconventional thermal cure cycle to the end of the printing process alsoincreases production time by more than 1 hour.

The problem is there is a lack of thermoset compositions that can curerapidly during printing to provide structures that do not delaminate.Accordingly, there is a need for a self-curing thermoset composition forprinting methods that do not require external irradiation to providestructures of good mechanical integrity.

SUMMARY

This disclosure enables 3D-printing of high quality thermosets that cureduring the printing process. It also allows printing of free-standingstructures with no support material. Furthermore, it enables printing ofcontinuous thermoset fiber-reinforced composites, which are notprintable by other techniques.

The process uses a gelled solution of monomer and catalyst that isextruded from a print head onto a heated surface. As the printingcontinues, a propagating polymerization front forms and propagatesthrough the extruded gel, fully curing it in seconds. This can befurther combined with the extrusion of a fiber reinforcement to allow 3Dprinting of continuous fiber reinforced thermoset composites.

The process uses a highly exothermic curing reaction, such that theprinted material can cure itself without continuous external photo orthermal stimulus. The process uses a printing strategy involving theextrusion of a partially cured resin in combination with a rapid curingtechnique based on frontal polymerization (FP). Frontal polymerization(FP) is a self-propagating reaction that is driven by the heat of anexothermic polymerization reaction. A partially cured frontallypolymerizable resin is extruded from a print head optionally containinga fiber reinforcing element. A small thermal stimulus ignites FP byactivating a latent initiator, which releases the heat ofpolymerization, which activates further initiator and ultimatelyproduces a propagating reaction wave that can quickly polymerize allavailable monomer. The polymerization wave propagates through theprinted material rapidly curing it with no further energy input. Nofurther treatment of the printed parts is required after FP is complete.This rapid curing is enabled by recent advances in frontalpolymerization chemistry.

Under specific conditions, the speed of the polymerization wave canself-equilibrate to the designed printing speed. This allows thematerial to cure immediately after extrusion even when the printingprocess uses varying print speeds, which has not possible withconventional 3D-printing. Alternatively, print speed and chemistry canbe matched to allow one layer to be deposited in the time it takes tocure the layer below it, enabling concurrent curing and printing at highspeed. This process should be much faster than other techniques forprinting fiber composites, while still producing high quality materials.Furthermore, it is easily adaptable to commercially available printingequipment, enabling widespread use.

Accordingly, this disclosure provides a composition for an organicphosphite, a ruthenium (II) catalyst, a ruthenium catalyst, acycloalkene, and an organic solvent, wherein the mixture forms athermoset gel having a storage modulus of about 10 Pascals to about10,000 Pascals.

This disclosure also provides a method for fabricating a printedthermoset polymer from the composition above, comprising:

-   -   a) extruding the thermoset gel from a dispensing apparatus to        provide an extruded thermoset gel; and    -   b) triggering a frontal polymerization of the extruded thermoset        gel, wherein the frontal polymerization propagates and cures the        extruded thermoset gel;

wherein a printed thermoset polymer is printed by steps comprising stepsa) and b).

Additionally, this disclosure provides a method for polymerizing athermoset gel comprising:

-   -   a) extruding a thermoset gel onto a platform from a cooled        dispensing apparatus to provide an extruded thermoset gel; and    -   b) heating the platform to trigger a frontal polymerization of        the extruded thermoset gel, wherein the frontal polymerization        propagates and cures the extruded thermoset gel;

wherein the thermoset gel comprises triethyl phosphite,dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](benzylidene)(tricyclohexylphosphine)ruthenium(II),and a dicyclopentadiene;

wherein the thermoset gel is thereby polymerized.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the specification and are includedto further demonstrate certain embodiments or various aspects of theinvention. In some instances, embodiments of the invention can be bestunderstood by referring to the accompanying drawings in combination withthe detailed description presented herein. The description andaccompanying drawings may highlight a certain specific example, or acertain aspect of the invention. However, one skilled in the art willunderstand that portions of the example or aspect may be used incombination with other examples or aspects of the invention.

FIG. 1. Chemical Structure of the 3D printable FROMP gel constituents.

FIG. 2. Schematics of setup for printing neat DCPD (left) andcarbon-fiber embed DCPD (right).

FIG. 3. Proof of TEP as an inhibitor.

FIG. 4. Viscosity vs. Time when TEP inhibitor is added.

FIG. 5. Propagation front in response to gel material at ambienttemperature (no equilibrating action). The plot show evolution of frontspeed when the print speed is high. Since the front is far behind theextruding nozzle, the gel material has warmed up to ambient temperature.The plot displays uninhibited frontal polymerization (FP) of gelmaterial.

FIG. 6. Propagation front in response to gel material of a lowertemperature (self-equilibrating action). The print speed is low suchthat the front is trailing right behind the extruding nozzle. FP for thegel with a lower temperature had capped the front velocity at theprinting speed.

FIG. 7. Thermal images were captured by an infrared camera while 3Dprinting neat DCPD. The front is evidenced by a distinct heated tip, dueto the exothermic nature of polymerization.

FIG. 8. Thermal image captured by an infrared camera while 3D printingneat DCPD. Captures the same propagating front as shown in FIG. 7 butthe image was captured 2 seconds later.

FIG. 9. Comparison of gel point and frontal velocity of differentphosphite inhibitors, including DMAP, TEP, and TBP, at between 0.3 and 8molar equivalents inhibitor to 100 ppm GC2.

FIG. 10. Example of a 3D printed structure—a helical spring. The imagesshow the spring at different points in time during the printing process.The printhead extrudes the partially cured gel, which is then almostimmediately cured by the propagating front. As a result, the printedhelix needs no support to maintain its shape during printing and isfully cured when printing is finished.

FIG. 11. Finished helical spring after completion of the printingprocess.

FIG. 12. Comparison of gel point and frontal velocity of differentphosphite inhibitors, including TMP, TEP, and TBP, at between 0.3 and 8molar equivalents inhibitor to 100 ppm GC2.

DETAILED DESCRIPTION

This disclosure provides a new process of 3D printing using frontalpolymerization (FP) chemistry that enables 3D-printing of tough, highquality thermosets in a short time with the option of adding fiberreinforcement. As such, it facilitates fabrication of mechanicallyrobust 3D-printed devices and structures. Minimal infrastructure isrequired to support this process, as it can be adapted to standardextrusion printing devices.

For FP chemistry to be amenable to 3D-printing, it must fulfill severalrequirements. First, since a microscale filament is undergoing FP, therewill be substantial heat loss to the environment during reaction. Thus,the reaction must be extremely exothermic and react quickly to allow theFP to propagate despite this heat loss. This rules out frontallypolymerized engineering thermosets such as epoxy, which do not reactquickly enough. Second, the FP'ed material must be robust enough to actas a structural polymer. Highly reactive acrylate FP would likelyprovide a workable printing solution, but it produces a brittle polymerthat would not serve well for a composite matrix for example. Finally,the reactive FP solution must be able to maintain a viscous gel-likestate for the entire printing period, such that it can be extruded froma 3D-printing nozzle, while remaining amenable to FP after extrusion.Together, these requirements seem to have prevented any development ofFP-related 3D-printing solutions.

Definitions

The following definitions are included to provide a clear and consistentunderstanding of the specification and claims. As used herein, therecited terms have the following meanings. All other terms and phrasesused in this specification have their ordinary meanings as one of skillin the art would understand. Such ordinary meanings may be obtained byreference to technical dictionaries, such as Hawley's Condensed ChemicalDictionary 14^(th) Edition, by R. J. Lewis, John Wiley & Sons, New York,N.Y., 2001.

References in the specification to “one embodiment”, “an embodiment”,etc., indicate that the embodiment described may include a particularaspect, feature, structure, moiety, or characteristic, but not everyembodiment necessarily includes that aspect, feature, structure, moiety,or characteristic. Moreover, such phrases may, but do not necessarily,refer to the same embodiment referred to in other portions of thespecification. Further, when a particular aspect, feature, structure,moiety, or characteristic is described in connection with an embodiment,it is within the knowledge of one skilled in the art to affect orconnect such aspect, feature, structure, moiety, or characteristic withother embodiments, whether or not explicitly described.

The singular forms “a,” “an,” and “the” include plural reference unlessthe context clearly dictates otherwise. Thus, for example, a referenceto “a compound” includes a plurality of such compounds, so that acompound X includes a plurality of compounds X. It is further noted thatthe claims may be drafted to exclude any optional element. As such, thisstatement is intended to serve as antecedent basis for the use ofexclusive terminology, such as “solely,” “only,” and the like, inconnection with any element described herein, and/or the recitation ofclaim elements or use of “negative” limitations.

The term “and/or” means any one of the items, any combination of theitems, or all of the items with which this term is associated. Thephrases “one or more” and “at least one” are readily understood by oneof skill in the art, particularly when read in context of its usage. Forexample, the phrase can mean one, two, three, four, five, six, ten, 100,or any upper limit approximately 10, 100, or 1000 times higher than arecited lower limit.

As will be understood by the skilled artisan, all numbers, includingthose expressing quantities of ingredients, properties such as molecularweight, reaction conditions, and so forth, are approximations and areunderstood as being optionally modified in all instances by the term“about.” These values can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings of the descriptions herein. It is also understood that suchvalues inherently contain variability necessarily resulting from thestandard deviations found in their respective testing measurements. Whenvalues are expressed as approximations, by use of the antecedent“about,” it will be understood that the particular value without themodifier “about” also forms a further aspect.

The terms “about” and “approximately” are used interchangeably. Bothterms can refer to a variation of ±5%, ±10%, ±20%, or ±25% of the valuespecified. For example, “about 50” percent can in some embodiments carrya variation from 45 to 55 percent, or as otherwise defined by aparticular claim. For integer ranges, the term “about” can include oneor two integers greater than and/or less than a recited integer at eachend of the range. Unless indicated otherwise herein, the terms “about”and “approximately” are intended to include values, e.g., weightpercentages, proximate to the recited range that are equivalent in termsof the functionality of the individual ingredient, composition, orembodiment. The terms “about” and “approximately” can also modify theend-points of a recited range as discussed above in this paragraph.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges recited herein also encompass any and all possible sub-ranges andcombinations of sub-ranges thereof, as well as the individual valuesmaking up the range, particularly integer values. It is thereforeunderstood that each unit between two particular units are alsodisclosed. For example, if 10 to 15 is disclosed, then 11, 12, 13, and14 are also disclosed, individually, and as part of a range. A recitedrange (e.g., weight percentages or carbon groups) includes each specificvalue, integer, decimal, or identity within the range. Any listed rangecan be easily recognized as sufficiently describing and enabling thesame range being broken down into at least equal halves, thirds,quarters, fifths, or tenths. As a non-limiting example, each rangediscussed herein can be readily broken down into a lower third, middlethird and upper third, etc. As will also be understood by one skilled inthe art, all language such as “up to”, “at least”, “greater than”, “lessthan”, “more than”, “or more”, and the like, include the number recitedand such terms refer to ranges that can be subsequently broken down intosub-ranges as discussed above. In the same manner, all ratios recitedherein also include all sub-ratios falling within the broader ratio.Accordingly, specific values recited for radicals, substituents, andranges, are for illustration only; they do not exclude other definedvalues or other values within defined ranges for radicals andsubstituents. It will be further understood that the endpoints of eachof the ranges are significant both in relation to the other endpoint,and independently of the other endpoint.

One skilled in the art will also readily recognize that where membersare grouped together in a common manner, such as in a Markush group, theinvention encompasses not only the entire group listed as a whole, buteach member of the group individually and all possible subgroups of themain group. Additionally, for all purposes, the invention encompassesnot only the main group, but also the main group absent one or more ofthe group members. The invention therefore envisages the explicitexclusion of any one or more of members of a recited group. Accordingly,provisos may apply to any of the disclosed categories or embodimentswhereby any one or more of the recited elements, species, orembodiments, may be excluded from such categories or embodiments, forexample, for use in an explicit negative limitation.

The term “contacting” refers to the act of touching, making contact, orof bringing to immediate or close proximity, including at the cellularor molecular level, for example, to bring about a physiologicalreaction, a chemical reaction, or a physical change, e.g., in asolution, in a reaction mixture.

A “sufficient amount” refers to an amount of time to bring about arecited effect, such as an amount necessary time between extrusion andfrontal polymerization to form a printed structure. Determination of asufficient amount is typically within the capacity of persons skilled inthe art, especially in light of the detailed disclosure provided herein.Thus, a “sufficient amount” generally means an amount of time thatprovides the desired effect.

The term “substantially” as used herein, is a broad term and is used inits ordinary sense, including, without limitation, being largely but notnecessarily wholly that which is specified. For example, the term couldrefer to a numerical value that may not be 100% the full numericalvalue. The full numerical value may be less by about 1%, about 2%, about3%, about 4%, about 5%, about 6%, about 7%, about 8%, about 9%, about10%, about 15%, or about 20%.

A “solvent” as described herein can include water or an organic solvent.Examples of organic solvents include hydrocarbons such as toluene,xylene, hexane, and heptane; chlorinated solvents such as methylenechloride, chloroform, and dichloroethane; ethers such as diethyl ether,tetrahydrofuran, and dibutyl ether; ketones such as acetone and2-butanone; esters such as ethyl acetate and butyl acetate; nitrilessuch as acetonitrile; alcohols such as methanol, ethanol, andtert-butanol; and aprotic polar solvents such as N,N-dimethylformamide(DMF), N,N-dimethylacetamide (DMA), and dimethyl sulfoxide (DMSO).Solvents may be used alone or two or more of them may be mixed for useto provide a “solvent system”.

As used herein, the term “substituted” or “substituent” is intended toindicate that one or more (for example, 1-20 in various embodiments,1-10 in other embodiments, 1, 2, 3, 4, or 5; in some embodiments 1, 2,or 3; and in other embodiments 1 or 2) hydrogens on the group indicatedin the expression using “substituted” (or “substituent”) is replacedwith a suitable group known to those of skill in the art, provided thatthe indicated atom's normal valency is not exceeded, and that thesubstitution results in a stable compound.

The term “alkyl” refers to a branched or unbranched hydrocarbon having,for example, from 1-20 carbon atoms, and often 1-12, 1-10, 1-8, 1-6, or1-4 carbon atoms. As used herein, the term “alkyl” also encompasses a“cycloalkyl”. Alkyl can also be substituted with one or moresubstituents.

The term “cycloalkyl” refers to cyclic alkyl groups of, for example,from 3 to 10 carbon atoms having a single cyclic ring or multiplecondensed rings. Cycloalkyl groups include, by way of example, singlering structures such as cyclopropyl, cyclobutyl, cyclopentyl,cyclooctyl, and the like, or multiple ring structures such as adamantyl,and the like. Cycloalkyl can also be substituted with one or moresubstituents.

The term “aryl” refers to an aromatic hydrocarbon group derived from theremoval of at least one hydrogen atom from a single carbon atom of aparent aromatic ring system. The radical attachment site can be at asaturated or unsaturated carbon atom of the parent ring system. The arylgroup can have from 6 to 30 carbon atoms, for example, about 6-10 carbonatoms. The aryl can also be substituted with one or more substituents.

The term “frontal polymerization” refers a process in which thepolymerization reaction propagates through a vessel or a substance.There are three types of frontal polymerizations: thermal frontalpolymerization (TFP) that uses an external energy source to initiate thefront; photofrontal polymerization (PFP), in which the localizedreaction is driven by an external UV source; and isothermal frontalpolymerization (IFP), which relies on the Norrish-Trommsdorff, or geleffect, that occurs when monomer and initiator diffuse into a polymerseed (small piece of polymer). Thermal frontal polymerization beginswhen a heat source contacts a solution of monomer and a thermalinitiator or catalyst. Alternatively, a UV source can be applied if aphotoinitiator is also present. The area of contact (or UV exposure) hasa faster polymerization rate, and the energy from the exothermicpolymerization diffuses into the adjacent region, raising thetemperature and increasing the reaction rate in that location. Theresult is a localized reaction zone that propagates down the reactionvessel as a thermal wave.

The term “ring-opening metathesis polymerization (ROMP)” refers to atype of olefin metathesis chain-growth polymerization that producesindustrially important products. The driving force of the reaction isrelief of ring strain in cyclic olefins (e.g. norbornene orcyclopentene). Thus, “frontal ring-opening metathesis polymerization(FROMP)” entails the conversion of a monomer into a polymer via alocalized exothermic reaction zone that propagates through the couplingof thermal diffusion and Arrhenius reaction kinetics. The pot-life,get-time, and reaction kinetics can be controlled through variousmodifications of the polymerization chemistry.

The terms “pot life” and “working life” are often taken to mean the samething for mixtures comprising reactive monomers. Pot-life is the amountof time between the mixing of monomer and initiator or catalyst and thepoint at which frontal polymerization is no longer possible. It can alsorefer to the amount of time it takes for an initial viscosity of acomposition to double, or quadruple. Timing starts from the moment theproduct is mixed, and is measured at room temperature. Working life,refers to the amount of time a mixture remains low enough in viscositythat it can still be easily applied to a part or substrate in aparticular application. For that reason, working life can vary fromapplication to application, and even by the application method of thereactive mixture. Pot life can act as a guide in determining workinglife by providing a rough timeline of viscosity growth.

The term “rheology modifier” refers to adjusted formulations that canalter the flow behavior or viscosity of compositions depending on theapplication.

The term “thixotropic filler” refers to a substance that can be added tothe disclosed composition to give it thixotropic or shear thinningproperty. Certain gels or fluids that are thick, or viscous, understatic conditions will flow (become thin, less viscous) over time whenshaken, agitated, sheared or otherwise stressed.

The term “storage modulus” is defined as a measure of elasticity of amaterial. It is the measured stored energy, representing the elasticportion, and the loss modulus measures the energy dissipated as heat,representing the viscous portion. The storage (and loss modulus)indicate the stress response for a visco-elastic fluid in oscillatoryshear. If applying a shear strain-rate that is cosine; a viscous fluidwill have stress proportional to the shear strain-rate (Newtonian); andan elastic solid will have stress proportional to the shear strain(sine—integral of cosine). A visco-elastic response will be a mixture ofthe two. The storage modulus is the elastic solid like behavior (G′) andthe loss modulus is the viscous response (G″). These will cross-overwhen the frequency is equal to the reciprocal relaxation time.

Embodiments of the Invention

Enablement of 3D-printing of tough, high quality thermosets in a shorttime with the option of adding fiber reinforcement to facilitatefabrication of mechanically robust 3D-printed devices and structures isdescribed herein. Minimal infrastructure is required to support thisprocess, as it can be adapted to standard extrusion printing devices. Agelled solution of monomer and catalyst is extruded from a print headonto a heated surface. As the printing continues, a propagatingpolymerization front forms and propagates through the extruded gel thathas a composition which can fully cure it in as little as a few seconds,or longer if required. This composition can be further combined with theextrusion of a fiber reinforcement to allow 3D printing of continuousfiber reinforced thermoset composites. The concept of 3D-printing viafrontal polymerization through printed resin has been demonstrated forusing several formulations of a dicyclopentadiene resin, rutheniumcatalyst, and phosphite inhibitor. Freeform and layer-by-layer polymerstructures have been fabricated using this technique.

Accordingly, this disclosure provides various embodiments of acomposition for a thermoset gel comprising a mixture of an organicphosphite, a ruthenium (II) catalyst, a cycloalkene, and an organicsolvent, wherein the mixture forms a thermoset gel having a storagemodulus of about 10 Pascals to about 10,000 Pascals.

In additional embodiments, the organic phosphite comprises one or moreof P(OR)₃, wherein R is (C₁-C₁₀)alkyl (preferably (C₁-C₆)alkyl), or aryl(or a substituted aryl). In some other embodiments, the phosphitecomprises trimethyl phosphite, triethyl phosphite, tripropyl phosphite,triisopropyl phosphite, tri-n-butyl phosphite, tri-sec-butyl phosphite,tri-tert-butyl phosphite, triphenyl phosphite, or a combination thereof.

In other additional embodiments, the ruthenium catalyst comprisesdichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](benzylidene)(tricyclohexylphosphine)ruthenium(II) (GC2),dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](indenylidene) (tricyclohexylphosphine)ruthenium(II),dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](indenylidene)(triisopropylphosphite)ruthenium(II), or a combination thereof.

In yet other additional embodiments, the cycloalkene comprises anorbornene, an endo-dicyclopentadiene (DCPD), an exo-DCPD, acyclobutene, a cycloctene, a cyclooctadiene, a cyclooctatetraene, or acombination thereof. In additional embodiments, the organic solventcomprises an aryl organic solvent or an alkyl-substituted aryl organicsolvent. In some embodiments, the organic solvent comprises benzene,ethylbenzene, diethylbenzene, dichlorobenzene, cyclohexylbenzene,n-methylpyrrolidone, toluene, xylene, mesitylene, or a combinationthereof. In other embodiments, the organic solvent is polar, ornon-polar.

In other embodiments, the mixture comprises about 0.1 mole equivalentsto about 10 mole equivalents of the organic phosphite based on moles ofthe ruthenium catalyst in the mixture. In yet other embodiments, themolar equivalents of the phosphite is about 0.3 to about 8, about 0.5 toabout 5, or about 0.2 to about 2. In some other embodiments, thedisclosed composition further comprises a carbon fiber, a thixotropicfiller, a rheology modifier, or a combination thereof. In otherembodiments the composition comprises an anti-thixotropic filler. Insome additional embodiments, the storage modulus is about 50 Pascals toabout 5000 Pascals. In other embodiments, the storage modulus is about100 Pascals to about 1000 Pascals, about 80 Pascals to about 1500Pascals, or about 200 Pascals to about 1200 Pascals.

This disclosure also provides various embodiments of a method forfabricating a printed thermoset polymer comprising:

-   -   a) extruding the disclosed thermoset gel from a dispensing        apparatus to provide an extruded thermoset gel; and    -   b) triggering a frontal polymerization of the extruded thermoset        gel, wherein the frontal polymerization propagates and cures the        extruded thermoset gel;

wherein a printed thermoset polymer is printed by steps comprising stepsa) and b).

In some embodiments, the thermoset gel is cooled below 25° C. In otherembodiments, the dispensing apparatus is cooled below 25° C. In yetother embodiments, the thermoset gel or dispensing apparatus is cooledto about 25° C. to about −80° C., about 20° C. to about −40° C., about10° C. to about −20° C., or about 5° C. to about −30° C. In otherembodiments the dispensing apparatus is cooled by a refrigerant or aPeltier cooler. In some other embodiments, the thermoset gel is storedin a cooler such as a refrigerator.

In additional embodiments, the extruded thermoset gel is extruded onto aplatform that can trigger frontal polymerization. In yet otherembodiments, the platform is heated. In additional embodiments, thefrontal polymerization is triggered at a temperature (or heated platformtemperature) of about 30° C. to about 100° C., about 50° C. to about150° C., about 75° C. to about 180° C., about 50° C. to about 200° C.,about 100° C. to about 250° C., or about 120° C. to about 300° C., Inother embodiments, the platform is a glass or metal plate or slide thatis heated with a heating element or heating device. In yet additionalembodiments, frontal polymerization is triggered by a chemicalinitiator, or electromagnetic irradiation, such as ultraviolet light.

In various embodiments, the frontal polymerization propagates and curesthe extruded thermoset gel instantly (about 1 second or less than onesecond) to about 60 minutes after the thermoset gel is extruded. Inother embodiments, the frontal polymerization propagates and cures theextruded thermoset gel a sufficient amount of time after the thermosetgel is extruded. In yet other various embodiments, the frontalpolymerization is delayed by a sufficient amount of time to allow asecond layer of extruded thermoset gel to coalesce or crosslink with afirst layer of extruded thermoset gel. In yet other embodiments thesufficient amount of time is about 0.01 seconds to about 5 hours, about1 second to about 2 hours, about 1 second to about 1 hour, about 1second to about 50 minutes, about 1 second to about 40 minutes, about 1second to about 30 minutes, about 1 second to about 20 minutes, about 1second to about 10 minutes, or about 1 second to about 5 minutes.

Additionally, this disclosure also provides various embodiments of amethod for polymerizing a thermoset gel comprising:

-   -   a) extruding a thermoset gel onto a platform from a cooled        dispensing apparatus to provide an extruded thermoset gel; and    -   b) heating the platform to trigger a frontal polymerization of        the extruded thermoset gel, wherein the frontal polymerization        propagates and cures the extruded thermoset gel;

wherein the thermoset gel comprises triethyl phosphite,dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](benzylidene)(tricyclohexylphosphine)ruthenium(II),and a dicyclopentadiene;

wherein the thermoset gel is thereby polymerized (i.e., fabricated).

In some embodiments, the thermoset gel comprises an additive to lowerthe melting point of the dicyclopentadiene. In some other embodiments,the additive is a norbornene. In other embodiments the norbornene is5-ethylidene norbornene.

In some embodiments the melting point is lowered by about 1° C. to about30° C., or about 2° C. to about 15° C. In other embodiments, the amountof the additive is about 0.1 weight % to about 15 weight %, relative tothe weight of the cycloalkane. In yet other embodiments, the amount ofadditive is 1 wt. % to 10 wt. %, or 2 wt. % to 8 wt. %. In yet otherembodiments, the thermoset gel has a storage modulus of about 50 Pascalsto about 5,000 Pascals. In additional embodiments, the extrudedthermoset gel is a carbon fiber composite.

This disclosure provides ranges, limits, and deviations to variablessuch as volume, mass, percentages, ratios, etc. It is understood by anordinary person skilled in the art that a range, such as “number1” to“number2”, implies a continuous range of numbers that includes the wholenumbers and fractional numbers. For example, 1 to 10 means 1, 2, 3, 4,5, . . . 9, 10. It also means 1.0, 1.1, 1.2. 1.3, . . . , 9.8, 9.9,10.0, and also means 1.01, 1.02, 1.03, and so on. If the variabledisclosed is a number less than “number10”, it implies a continuousrange that includes whole numbers and fractional numbers less thannumber10, as discussed above. Similarly, if the variable disclosed is anumber greater than “number10”, it implies a continuous range thatincludes whole numbers and fractional numbers greater than number10.These ranges can be modified by the term “about”, whose meaning has beendescribed above.

Adapting FP chemistry to 3D-printing Frontal ring-opening metathesispolymerization via ruthenium catalysis has previously been shown to bean effective means for quickly producing polydicyclopentadiene (PDCPD).Polydicyclopentadiene (PDCPD) is a material of growing importance in thefabrication of durable parts due to its low density; high toughness,impact strength, and stiffness; and chemical resistance.^([1]) There isalso interest in using it for fabricating fiber-reinforcedcomposites.^([2]) Previous FROMP chemistry had not been useful for3D-printing, due to its lack of stability caused by the rutheniumcatalyst's high activity toward the DCPD monomer. Inhibitors such astriphenyl phosphine and dimethylaminopyridine (DMAP), which were shownin the literature to completely suppress catalysis in other systems,only increased the pot lives of liquid DCPD/catalyst mixtures to, atmost, 30 minutes. After this time, the material would either undergorapid spontaneous exothermic polymerization or would gel and shortlyafterward become unable to achieve FROMP. Furthermore, these inhibitorssignificantly reduced the frontal velocity, to the point where FROMPwould not occur.

Alkyl phosphites are a class of inhibitors for FROMP that dramaticallyextend pot life, while minimally affecting frontal velocity. There hasbeen little exploration of phosphites as inhibitors for ROMP catalysts,probably because of how much they hinder reaction at room temperature.In the past 5 years, there have been a few examples of catalystssynthesized with phosphite ligands that exhibited good high-temperatureactivity in ring-opening metathesis reactions not seen at lowtemperature.^([3-5]) These results indicated phosphites might exhibitgood characteristics as inhibitors for FROMP, since they would bindstrongly enough to the ruthenium to prevent room temperature reaction,but not strongly enough to prevent high temperature reactions.

Initial experiments showed phosphites to be far superior to previouslyused DMAP. Low concentrations (0.3-8 eq. to catalyst) oftrimethylphosphite (TMP), triethylphosphite (TEP), and tributylphosphite(TBP) were shown to extend gelation from 12 minutes to >30 h while stillenabling FP.

The key, enabling discovery for 3D-printing was that there was anextended processing window after gelation wherein the material was stillfrontally polymerizable. This had never been observed for DMAP-inhibitedFROMP, since the material reacted too fast after gelation. However, withthe potent phosphite inhibitors, the “reactive gel” period was extendedto several hours or more. Thus, partially cured gel-FROMP solution couldbe used as a 3D-printing ink that could undergo FP immediately afterprinting to produce an engineering thermoset polymer. Characterizing thecuring process rheologically, enabled a determination of precisely whenthe material would reach the desired viscosity for printing. (FIG. 4)Processing was further enabled by freezing the gel-FROMP solutions assoon as they reached this point, essentially halting reaction, andallowing for printing of the B-stage PDCPD on demand.

Since FROMP of DCPD releases thermal energy very quickly, it can occurin single gel filaments extruded by the print head. Since there is arapid transition from gel to solid, the material is self-supporting,requiring no secondary material to facilitate printing, as is the casewith traditional thermoplastic 3D-printing systems. Instead, a freeformstructure can be printed in whatever geometry is desired.

While we have not yet demonstrated this, a layer-by-layer approachshould also be feasible, wherein a front propagates from the ground up,while the printer lays down layers of material at a commensurate rate.Thus, each gel layer should be able to merge with the previous onebefore being frontally polymerized.

The current strategy focuses entirely on printing the partially curedDCPD material, since it possesses a desirably high viscosity. Thisstrategy was chosen because of its simplicity, since it allows for asingle-component system. However, the material's viscosity will changeover time as it cures at room temperature. This factor is currentlymitigated by cooling the printing barrel, but there is another solutionthat should also be feasible. Thixotropic fillers are well known toincrease viscosity at relatively low loadings. By incorporating these,we could print immediately after mixing up the solution and, thus,extend the processing window.

The frontal velocity is also a function of the gel temperature. Thelower gel temperature causes the propagating front to decelerate andmatch the printing speed. FIG. 5 and FIG. 6 shows front decelerationnaturally occurring in response to a lower print speed. This phenomenonallows the system to intelligently tune the curing process to variableprint speeds, which is inevitable in the manufacturing of complexarchitectures.

It is also possible to incorporate continuous carbon fiber reinforcementinto the printed material. It was previously demonstrated that carbonfiber accelerates FP through its thermal conductivity (see U.S. patentapplication Ser. No. 15/462,458 filed Mar. 11, 2017). Since the curingprocess is thermally driven, the addition of a carbon fiber tow to theprinted gel solution can actually accelerate the printing process.

In conclusion, the development of alkyl-phosphites as potent FROMPinhibitors slows the curing process of DCPD substantially, such that itremains a viscous gel for several hours. This gel can be extruded from a3D print head and immediately FROMPed to form structural PDCPD thatrequires no supporting material. In this way, complex 3D structures canbe printed quickly and easily. Furthermore, the nature of the curingreaction facilitates 3D-printing of continuous fiber-reinforcedcomposites, which are typically challenging to print.

The following Example is intended to illustrate the above invention andshould not be construed as to narrow its scope. One skilled in the artwill readily recognize that the Examples suggest many other ways inwhich the invention could be practiced. It should be understood thatnumerous variations and modifications may be made while remaining withinthe scope of the invention.

EXAMPLE Example. Materials and Methods Formulation and Technique for3D-Printing Via Frontal Polymerization

The 3D printable FROMP gel consists of three main components: themonomer, catalyst and inhibitor. A combination of different chemicalscan be used for this technique. As such, this approach could be broadlyapplicable to several thermally triggered polymerizations.

Dicyclopentadiene (DCPD) and Grubbs catalyst, 2nd generation (GC2) areused as the monomer and catalyst respectively. A number of compoundswere found useful for inhibiting the reactive chemistry between DCPD andGC2. This includes trimethyl phosphite (TMP), triethyl phosphite (TEP),triisopropyl phosphite (TIPP), tributyl phosphite (TBP), triphenylphosphite (TPP) and 4-dimethylaminopyridine (DMAP).

To prepare the gel, DCPD is melted in an oven at 35° C. and combinedwith 5 wt. % 5-ethylidene norbomene to lower the melting point belowroom temperature. GC2 (1.00 mg) is dissolved in 111 μl of anhydroustoluene. TEP (0.100 μl) is then mixed into the catalyst solution, andsubsequently added to 1.551 g of the DCPD solution. The final solutionis transferred into 3 cc Optimum syringe barrels from Nordson EFD. Thesyringe barrels are then left to stand at 23.0° C. between 160 minutesto 240 minutes such that the DCPD slowly polymerizes to become a gel.

Setup for 3D Printing

Schematic of both 3D printing setups are illustrated in FIG. 2. Syringebarrels and dispense tips are used (Nordson EFD, Optimum® generalpurpose tips). This setup is mounted on a robotic motion-controlledstage (Model JL2000, Robocasting Enterprises). A custom-designedsoftware (RoboCAD 2.0) may be programmed to execute specific sets ofstage motion while delivering pressurized air. Air is supplied from acompressed air tank via an air-dispensing system (Model 800, NordsonEFD). The setup is cooled down to −5° C. using a Peltier cooler.

Printing Neat DCPD

The syringe barrel is filled two-thirds with DCPD gel then sealed with apiston (Nordson EFD, Optimum® cartridge pistons). It is then placed inan air-operated high-pressure dispensing tool, HP10 cc (Nordson EFD).The HP10 cc uses pressurized air (up to 2.76 MPa) to actuate a plungerthat pushes down onto the seal. The compression extrudes DCPD gelthrough the dispense tip which nozzle diameters range from 0.10 mm to1.54 mm.

Printing Carbon Fiber Embed DCPD

For this setup, a Teflon seal is custom-designed to keep the pressurizedgel from leaking while still enabling delivery of continuous carbonfiber. It has an off-center through hole which sits a O-ring (AppleRubber, MicrOring® Seal) of inner diameter of 0.40 mm. Continuous carbonfiber (Hexcel, HexTow® AS4C-3K) is threaded through the O-ring and thedispense tips.

The syringe barrel is filled two-thirds with DCPD gel then sealed withthe custom seal. It is then placed in HP10 cc to perform the sameextrusion motion. The compression extrudes carbon fiber embed DCPDthrough the dispense tip.

While specific embodiments have been described above with reference tothe disclosed embodiments and examples, such embodiments are onlyillustrative and do not limit the scope of the invention. Changes andmodifications can be made in accordance with ordinary skill in the artwithout departing from the invention in its broader aspects as definedin the following claims.

All publications, patents, and patent documents are incorporated byreference herein, as though individually incorporated by reference. Nolimitations inconsistent with this disclosure are to be understoodtherefrom. The invention has been described with reference to variousspecific and preferred embodiments and techniques. However, it should beunderstood that many variations and modifications may be made whileremaining within the spirit and scope of the invention.

What is claimed is:
 1. A composition for a thermoset gel comprising amixture of an organic phosphite, a ruthenium(II) catalyst, acycloalkene, and an organic solvent, wherein the mixture forms athermoset gel having a storage modulus of about 10 Pascals to about10,000 Pascals.
 2. The composition of claim 1 wherein the organicphosphite comprises one or more of P(OR)₃, wherein R is (C1-C10)alkyl oraryl.
 3. The composition of claim 1, wherein the organic phosphitecomprises trimethyl phosphite, triethyl phosphite, tripropyl phosphite,triisopropyl phosphite, tri-n-butyl phosphite, tri-sec-butyl phosphite,tri-tert-butyl phosphite, triphenyl phosphite, or a combination thereof4. The composition of claim 1 wherein the ruthenium (II) catalystcomprisesdichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](benzylidene)(tricyclohexylphosphine)ruthenium(II)(GC2),dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](indenylidene)(tricyclohexylphosphine)ruthenium(II),dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](indenylidene)(triisopropylphosphite)ruthenium(II), or a combination thereof.
 5. Thecomposition of claim 1 wherein the cycloalkene comprises a norbornene, adicyclopentadiene, a cyclobutene, a cycloctene, a cyclooctadiene, acyclooctatetraene, or a combination thereof.
 6. The composition of claim5 wherein the cycloalkene comprises the norbornene and thedicyclopentadiene, the norbornene being included in the mixture to lowera melting point of the dicyclopentadiene.
 7. The composition of claim 1wherein the organic solvent comprises an aryl organic solvent or analkyl-substituted aryl organic solvent.
 8. The composition of claim 1wherein the mixture comprises about 0.1 mole equivalents to about 10mole equivalents of the organic phosphite based on moles of theruthenium catalyst in the mixture.
 9. The composition of claim 8,wherein the mixture comprises about 0.3 to about 0.8 mole equivalents ofthe organic phosphite based on moles of the ruthenium catalyst in themixture.
 10. The composition of claim 1 wherein the storage modulus isabout 50 Pascals to about 5000 Pascals.
 11. The composition of claim 1wherein the thermoset gel comprises a gelation stability of greater than30 hours.
 12. The composition of claim 1 wherein the mixture isconfigured to undergo frontal polymerization triggered by heat, achemical initiator, or electromagnetic radiation.
 13. The composition ofclaim 12, wherein the electromagnetic radiation comprises ultravioletlight.
 14. The composition of claim 1 wherein the mixture furthercomprises a carbon fiber, a thixotropic filler, a rheology modifier, ora combination thereof.
 15. The composition of claim 14 wherein a carbonfiber composite is formed upon curing the thermoset gel.